1. Field of the Invention
The present invention relates to a strip threading speed controlling apparatus for a tandem rolling mill.
2. Related Background Art
In a hot tandem rolling mill, a target value of roll gaps and a rolling mill speed target value are calculated in consideration of characteristics of materials and rolling conditions as well in order to attain a desired strip thickness, a desired strip width and a desired rolled material temperature, and are set as initial values. These processes are executed by a set up calculation function.
An outline of the conventional set up calculation function in the tandem rolling mill will be explained referring to FIG. 4. A rolled material 1 is fed sequentially through a series of rolling stands 2a, 2b, 2c, - - - , 2n disposed in tandem, and is subjected to a rolling process. The rolling stands 2a 2n are provided with roll gap adjusters 3a, 3b, 3c, 3n. Work rolls of the rolling stands 2a-2n are rotationally driven by electric motors 4a, 4b, 4c, - - - , 4n, respectively. Speeds of the electric motors 4a-4n are controlled by speed controllers (ASR) 5a, 5b, 5c, - - - , 5n so as to attain a predetermined speed of the rolling mill. Each of loopers 6a, 6b, 6c, - - - , 6m for controlling an interstand rolled material tension is provided between the two adjacent rolling stands. The loopers 6a, 6b, 6c, - - - , 6m are provided with tension meters 7a, 7b, - - - , 7m, respectively, for measuring a tension of the rolled material 1. A setup calculation device 9 gives a speed command via a set up execution device 8 to the speed controllers 5a-5n. The set up calculation device 9 calculates a roll speed target value and a roll gap target value for each rolling stand in accordance with a rolling condition and target values of a thickness and a width of the rolled material, which are given each time. The roll speed target value is given as a speed command to the controllers 5a-5n via the set up execution device 8. The roll gap target value is given to the roll gap adjusters 3a-3n similarly via the set up execution device 8 and an unillustrated signal route.
A procedure of the set up calculation made by the set up calculation device 9 will be explained referring to FIG. 5. Generally in the hot rolling, to start with, a roll speed of the rolling stand serving as a pivot (reference) is calculated in order to set a temperature of the rolled material on an delivery side of the last rolling stand to a desired value (block 502). In general, the last rolling stand is set as the pivot stand. On the other hand, with a start of the set up calculation, a predictive calculation of a rolling force is performed, and a predictive value thereof is allocated to each rolling stand (block 504). Further, a strip thickness on the delivery side of each rolling stand is calculated (block 506), and thereafter a forward slip is calculated (block 508). With reference to results of those calculations, a speed of each rolling stand is calculated so that a mass flow (=width*thickness*speed=material moving quantity per unit time) of each rolling stand becomes constant (block 510). The speed, the forward slip, the delivery-side strip thickness and the rolling force necessary for the set up calculation influence on each other, and hence a convergence calculation might be performed in some cases as the necessity may arise in the block 503 which is indicated by the dotted line and embraces the blocks 504-510. After calculating the speed of each rolling stand, the roll gap of each rolling stand is calculated (block 512), and the set up calculation comes to an end.
In the thus executed set up calculation, there might occur an error because of implementing the predictive calculation based on a model etc. Automatic Gage Control, Looper Control for tension control, and control of temperature of the rolled material by water cooling, are carried out for eliminating the above error and further an influence of disturbance after starting the rolling process.
It is presumed in the set up calculation described above that front and back tensions at each rolling stand become a steady state as the target values indicate. In this case, there is induced such a state that a mass flow in the rolling stands disposed upstream is smaller than a mass flow in the rolling stands disposed downstream. As a result, the tension between the rolling stands might increase in the great majority of cases. The reason for this is elucidated as follows.
It is a general notion that the forward slip is largely influenced by a draft as well as being influenced by the front and back tensions. Based on the generality, the forward slip can be modeled by the formula (1): EQU f.sub.i=f.sup.0.sub.i +.alpha..sub.fi (t.sub.fi /K.sub.mi)-.beta..sub.fi (t.sub.bi /K.sub.mi) ... (1)
however, f.sup.0.sub.i =.alpha..sub.fi r.sub.i.sup.bfi, and r.sub.i =1-h.sub.i /H.sub.i where i is the rolling stand number, f.sub.i is the forward slip, t.sub.fi is the front tension, t.sub.bi is the back tension, K.sub.mi is the rolled material deformation resistance, r.sub.i is the reduction, h.sub.i is the delivery-side strip thickness, H.sub.i is the entry-side strip thickness, .alpha..sub.fi, .beta..sub.fi, a.sub.fi, b.sub.fi are the positive coefficients.
The stand speed V.sub.Ri-1 is calculated based on the following formula (2) by use of the forward slip f.sub.i because of the mass flow being constant. EQU h.sub.i-1 V.sub.Ri-1 1+f.sub.i-1)=h.sub.i V.sub.Ri (1+f.sub.i) ... (2)
Namely, the delivery-side strip thickness h.sub.i at each rolling stand is determined, and, if the speed V.sub.Ri at the reference rolling stand (the pivot stand) is determined, it follows that the speed V.sub.Ri-1 at the (i-1)- th stand adjacent upstream is determined. Note that generally a speed for setting the temperature of the rolled material on the delivery side of the last stand as the target value indicates, is selected as a speed at the pivot stand.
As shown in FIG. 6, with an emphasis on the rolling stand 2b, e.g., after the rolled material 1 comes out of the rolling stand 2b and before being bitten in by the next rolling stand 2c, a rolled material tension on the delivery side of the stand 2b, i.e., the front tension t.sub.f2 at the stand 2b is t.sub.f2 =0. The forward slip f.sub.2 of the rolled material just under the stand 2b in this case becomes, based on the formula (1), smaller than the forward slip when the front tension acts. Therefore, a rolled material speed V.sub.S1 between the stand 2b and the stand 2c when the front tension does not act, is smaller than a rolled material speed V.sub.s2 between the stand 2b and the stand 2c when the front tension acts.
In such a case, however, the conventional set up calculation has hitherto involved the use of the interstand rolled material speed V.sub.s2 when the front tension acts, and hence the rolled material speed is estimated larger than the actual speed V.sub.s1 immediately after threading the strip. The speed of the electric motor of the stand 2b is set to a much smaller value. As a result, the interstand tension excessively increases after threading the strip. When the tension is too large, the strip thickness becomes excessively thin, and the strip width becomes excessively small, with the result that a high-quality rolled material is hard to obtain. Further, if the quality declines, the rolled material might be fractured due to an over-tension, resulting in hindrance against a stable operation.